The present invention relates to an apparatus for controlling the beam shape of neutron beam, the velocity direction thereof or the like, and to an apparatus for measuring the energy of neutrons.
Neutron is an important probe in material science due to its features such as the follows: (a) it interacts with nuclei in a material strongly; (b) it has kinetic energy and wavelength at the same order as the atomic motion in a material and the scale of atomic structure, respectively; and (c) it has a strong penetrability.
Neutrons provide the information of nuclei in a material through the nuclear interaction, while X-rays and photons provide the information of atoms in a material through the electromagnetic interactions. Therefore, neutron scattering experiments are necessary in the determination of the position and the motion of nuclei regardless of electron clouds of atoms.
The strength of neutron-nuclear interaction has irregularity with respect to the atomic number of elements and varies depending on the mass number of isotopes, which is largely different from the strength of electromagnetic interaction which has a monotonous dependence only on the atomic number. This feature is applied to distinguish elements having similar electromagnetic scattering strengths and isotopes of an atomic number. It is also applicable to determine the position and the motion of light elements, for example, for the study of hydrogen atoms in an organic material.
The neutron magnetic dipole moment originated from its xc2xd spin is suitable to study the magnetic structure of a material. The strong penetrability can be applied to investigate the macroscopic structure of bulk samples such as industrial products, which are difficult to be investigated using charged particles and X-rays.
Efficient use of neutron beam is very important since neutron beam is available only in limited facilities equipped with nuclear reactors, accelerators and strong radioactive sources. Improvement of neutron beam transport from a neutron source to a neutron spectrometer is strongly desired since the improvement of neutron source intensity is limited by both the cost and the radiation control technique utilized. The improvement not only reduces the measuring time but also enables us to carry out in situ measurements of transient phenomena and to study the structure of new materials which are not available in the form of large-scale single crystals. It also reduces the risks in radiation safeties.
A neutron guide has been commonly used for transport of neutrons. A neutron beam can be bent by the reflection on the interface of medium (e.g., the interface between air and other medium) with a sufficiently small incident angle. A neutron guide is a vacuum tube of which inner surface is coated with a neutron reflector such as nickel. The neutron guide is pumped to vacuum to minimize the neutron loss caused by, for example, scattering by air. Neutrons incident to the guide at an angle smaller than the critical angle of the neutron reflector material are reflected on the inner surface and transported downstream.
FIG. 16A illustrates the concept of a prior art apparatus for the structural analysis of a material by neutron scattering, and FIG. 16B is the enlarged view of the apparatus around the sample. Neutrons are emitted in all directions from a neutron source 100 (e.g., a nuclear reactor, a radioactive source or a nuclear target bombarded by charged particles). A part of the generated neutrons are transported through a neutron guide 101 and incident to a sample 102. A neutron detector 103 such as a proportional counter 103 measures the intensity of neutrons scattered to the direction of angle xcex8. Angular distribution of scattered neutrons is analyzed to extract the information related to the atomic structure of the sample. The typical aperture of the neutron guide 101 is about 5 cm and the typical size of the sample 102 is 1-2 cm or larger.
For the purpose of in creasing the beam density, a neutron capillary tube can be used. The neutron capillary tube may be in the form of bundled glass tubes 110 which have thin channels of about 10 xcexcm in diameters as shown in FIG. 17. Incident neutrons are transported by the reflection on the inner surface of the channels. Each of the tubes 110 may be bent at a certain angle to focus the neutrons in the tube 110 to a small area 113, whereby the neutron beam density is increased.
Beam divergence of the incident beam should be sufficiently small to provide a good resolution in determination of scattering angles, since the scattering angle cannot be determined precisely if the incident beam is divergent. A common technique for reducing the beam divergence is the neutron diffraction. However, this technique has such a disadvantage that the intensity of the beam transferred to the sample is too much attenuated upon the diffraction.
In the analysis of a new material using neutron beam as a prove, only a very small sample is available. Such a very small sample needs to be irradiated with a dense and thin neutron beam for yielding good analysis result. Moreover, the incident neutron beam is also required to have a small beam divergence for the determination of the atomic structure of the sample with great precision.
The prior art neutron guide, which utilizes the total reflection of neutron beam on the interface between media, can transport neutrons efficiently but cannot focus nor reduce the divergence of the beam. As shown by the arrows in FIG. 16B, neutron beam 104 emitted from a neutron guide 101 diverges. The divergent neutron beam 104 enters a sample 102, from which neutrons each having various scattering angles of xcex81, xcex82, . . . are emitted to a detector 103 and detected by the detector 103. This causes a non-negligible error in determination of neutron scattering angles. In order to reduce the error, a beam collimator can be placed upstream the sample, which disadvantageously suppresses the efficiency of utilization of the neutrons.
As mentioned above, the use of neutron capillary tubes can increase the neutron beam density. However, as shown in FIG. 17, only the neutrons transported through the thin channels of the capillary tubes 110 are focused downstream and the neutrons 112 passing between tubes 110 are not used, which suppresses the efficiency of utilization of the neutrons. In addition, since tubes 110 are curved to bring neutrons into convergence, the beam divergence is enlarged at the focal point, which is not suitable for a good angular resolution.
To provide neutron beam with reduced divergence, neutron diffraction by a single crystal can be utilized. However, in this case, the beam intensity is too much attenuated.
As mentioned above, conventional methods related to neutron beam control are not appropriate to obtain a thin and dense neutron beam. In addition, up to now, no apparatus is known which can measure the energy of neutrons directly and conveniently.
Under these circumstances, the present invention has been accomplished. That is, an object of the present invention is to provide an apparatus for controlling the beam shape of neutron beam, the velocity direction thereof or the like. Another object of the present invention is to provide an apparatus for measuring the energy of neutrons directly and readily.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof.